1. Field of the Invention
The present invention relates to an image processing technique that corrects color blur of an image generated by image pickup.
2. Description of the Related Art
Images generated by image pickup performed by image pickup apparatuses such as digital cameras are often deteriorated by aberrations of an image taking optical system provided in the image pickup apparatus. In particular, longitudinal chromatic aberration of the image taking optical system causes color blur in a color image.
Since lights of mutually different wavelengths have mutually different refractive indices, these lights entering an image taking optical system form optical images at mutually different positions in an optical axis direction of the image taking optical system. Image-pickup elements each converting an object image (optical image) formed by the image taking optical system in the image pickup apparatus into electronic image information generally have a planar light-receiving surface, and only an optical image formed on the light-receiving surface is converted into sharp image information. Therefore, the longitudinal chromatic aberration generates, as the image information obtained by using the image-pickup element, an image including color blur generated by overlapping defocused color image components.
Japanese Patent Laid-Open No. 2008-85773 discloses a method for correcting color blur caused due to longitudinal chromatic aberration. This method obtains sharp (in-focus) images of respective wavelengths while changing a relative distance between an image taking optical system and an image-pickup element such that lights of the respective wavelengths form the sharp images on the image-pickup element, and combines these sharp images to obtain a color image in which the longitudinal chromatic aberration is corrected.
Moreover, so-called purple fringe also generates color blur, which deteriorates an image. The purple fringe is a phenomenon in which color blur of purple is generated near a high luminance area or the like in the image since point image distributions of respective wavelengths are mutually different due to longitudinal chromatic aberration, chromatic spherical aberration and chromatic comatic aberration. Japanese Patent Laid-Open No. 2006-115039 discloses a method for correcting such purple fringe. This method performs the correction by determining an area where the purple fringe is generated based on a distance from a luminance saturated area and closeness to chroma and hue of a specific color, and by performing a predetermined spatial operation on the determined area. This method uses a fact that the purple fringe is easily generated near the luminance saturated area and easily becomes a specific color of purple.
The method disclosed in Japanese Patent Laid-Open No. 2008-85773 combines the in-focus images of the respective wavelengths to correct the longitudinal chromatic aberration, which enables improvement of image quality. Moreover, the method disclosed in Japanese Patent Laid-Open No. 2006-115039 supposes the area where the purple fringe is generated and corrects the purple fringe to some degree on the basis of the supposition.
However, there are actually some cases where the methods disclosed in Japanese Patent Laid-Open Nos. 2008-85773 and 2006-115039 cannot correct the color blur well. For example, the methods cannot correct well color blur generated in an image obtained by image pickup for a three-dimensional object. Image pickup apparatuses generally focus on one plane (in-focus point) in an object space by an autofocus function or a manual focus operation and then perform image pickup. In such image pickup, in a case where an object including the plane is a three-dimensional object, object distances at respective angles of view are mutually different. In this case, a relatively sharp image of the in-focus point is obtained, but defocused images according to the object distances are obtained at out-of-focus points. Since the longitudinal chromatic aberration, the chromatic spherical aberration and the chromatic comatic aberration are varied according to the object distance, the color blur generated in the defocused image is varied according to a distance from the in-focus point. Such color blur being varied according to the object distance is hereinafter referred to as “color blur due to defocus”. The method disclosed in Japanese Patent Laid-Open No. 2008-85773 can correct the color blur due to the longitudinal chromatic aberration for only an in-focus image of an object located at a specific distance, but cannot correct the color blur due to defocus. The method disclosed in Japanese Patent Laid-Open No. 2006-115039 cannot provide a sufficient effect to correct the color blur due to defocus since the method utilizes a tendency of generation of the color blur at the in-focus point.